A wearer may be prescribed a positive or negative optical power correction. For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition representing the power increment between far vision and near vision. The addition is qualified as prescribed addition. Ophthalmic lenses suitable for presbyopic wearers are multifocal lenses, the most suitable being progressive multifocal lenses.
The ophthalmic prescription can include an astigmatism prescription. Such a prescription is produced by the ophthalmologist in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). The amplitude value represents the difference between minimal and maximal power in a given direction which enables to correct the visual defect of a wearer. According to the chosen convention, the axis represents the orientation of one of two powers with relation to a reference axis and in the sense of rotation chosen. Usually, the TABO convention is used. In this convention, the reference axis is horizontal and the sense of rotation is anticlockwise for each eye, when looking to the wearer. An axis value of +45° therefore represents an axis oriented obliquely, which when looking to the wearer, extends from the quadrant located up on the right to the quadrant located down on the left. Such an astigmatism prescription is measured on the wearer looking in far vision. The term <<astigmatism>> is used to designate the pair (amplitude, angle); despite this use not being strictly correct, this term is also used to refer to the amplitude of the astigmatism. The person skilled in the art can understand from the context which meaning is to be considered. It is also known for the person skilled in the art that the prescribed power and astigmatism of a wearer are usually called sphere, cylinder and axis. Ophthalmic lenses correcting the astigmatism prescription of a wearer may be composed of sphero-cylindrical surfaces.
To improve the optical characteristics (or optical performances) of an ophthalmic lens, method for optimizing methods of the parameters of the ophthalmic lens are thus used. Such optimization methods are designed so as to get the optical function of the ophthalmic lens as close as possible to a predetermined target optical function. FIG. 1 illustrates a schematic flowchart for carrying out such a method. The target optical function is determined at step 10. The target optical function represents the optical characteristics the ophthalmic lens should have. In the context of the present invention and in the remainder of the description, the term “target optical function of the lens” is used for convenience. This use is not strictly correct in so far as a target optical function has only a sense for a wearer—ophthalmic lens and ergorama system. Indeed, the optical target function of such system is a set of optical criteria defined for given gaze directions. This means that an evaluation of an optical criteria for one gaze direction gives an optical criteria value. The set of optical criteria values obtained is the target optical function. The target optical function then represents the performance to be reached. In the simplest case, there will only be one optical criterion such as optical power or residual astigmatism; however, more elaborate criteria may be used such as mean power which is a linear combination of optical power and astigmatism. Optical criteria involving aberrations of higher order may be considered. The number of criteria N considered depends on the precision desired. Indeed, the more criteria considered, the more the lens obtained is likely to satisfy the wearer's needs. However, increasing the number N of criteria may result in increasing the time taken for calculation. The choice of the number N of criteria considered will then be a trade-off between these two requirements. More details about target optical functions, optical criteria definition and optical criteria evaluation can be found in co-pending patent application EP-090305949 which was filed on Oct. 7, 2009 at the EPO. This optical function is used at step 12 of optical optimization. It results in an optimized lens as shown in the result box 14.
In some cases, even though the ophthalmic lens is optimized, the optical function of the optimized ophthalmic lens may not reach the target optical function. In some case, the optical function of the optimized ophthalmic lens may have optical characteristic worse than if the ophthalmic lens had not been optimized.
When calculating a pair of lenses according to the prior art, the lenses are calculated separately. The lens for the right eye is calculated from data relating to the right eye as the prescription of the right eye. Similarly, the lens for the left eye is calculated from data relating to the left eye as the prescription of the left eye.
Only monocular vision of each eye of the wearer is taken into account separately when designing the lenses of a pair. However, binocular vision is important in many of daily tasks as explained in Borish's Clinical Refraction Second Edition from the Author William J. Benjamin Chapter 5. Thus, it is important to take into account the two eyes when designing lenses of a pair to guaranty the natural behavior of the ocular couple.